SDR

RX888 External Clock Interface Kit

As you may know, the powerful RX888 SDR receiver is truly a game-changer — especially when used in conjunction with ka9q-radio and wsprdaemon software. This combination provides full HF-band coverage and analysis — much more than I can go into here.

But to obtain the desired frequency accuracy and stability necessary for serious propagation analysis the RX888 does need an external 27 MHz reference clock. Fortunately, the receiver provides a small “U.FL” connector on the board, allowing easy interface to the clock circuitry. However, it’s not as simple as connecting the output of a GPSDO (typically a Bodnar, or the upcoming TAPR reference). The RX888 clock input does not provide a 50 Ohm termination, and requires an AC-coupled reference of around 1V P-P. What is needed at this interface is a DC-block, an attenuator, and a termination.

Rather than string together a bunch of little adaptors, we (Turn Island Systems and TAPR) have come up a single-board solution, as well as a replacement back-panel (so you don’t have to drill a hole in the existing panel for an SMA jack), and a short U.FL jumper cable.

That SMA plug on the right-hand side of the adaptor is not installed for the “Inside the RX888” configuration, but the footprint is there so this board can be used as an external adaptor.

As you can see, the circuit is quite simple. Here’s the schematic:

And it’s pretty easy to cobble together an interface board yourself. Here’s my first attempt, which has been in operation for about a month now. This one isn’t exactly the same (it’s missing one capacitor and the attenuator values are slightly different):

I usually do my prototyping with surface-mount components, but for old-times sake I decided to use my old stash of through-hole parts on this. I’m waiting for delivery of the assembled boards, but if all goes as planned these kits should be available by the end of April.

If you want to use off-the-shelf components to interface between a Bodnar GPSDO and the RX888, I recommend inserting a DC-block at the RX888 external clock jack, followed by a 10 dB attenuator (the links are to probably-OK parts, I haven’t tried these particular ones myself). The attenuator reduces the output of the Bodnar to a more appropriate level, and provides a “good enough” termination at the RX888. Since the board-end of the internal jumper cable is unterminated the 10dB attenuator only provides about 8 dB of attenuation, and this is fine. The attenuator also provides a compromise termination to both the cable from the GPSDO and to the internal jumper.

Here is a good description of the do-it-yourself external clock modification to the RX888: http://www.sonic.net/~n6gn/ExtClockMod.pdf

Note that since Glenn wrote this we have discovered that the coupling / DC-block capacitor is desirable. Also, Glenn has included a switch that allows you to enable/disable the RX888 internal clock. Many of us are not bothering with the switch, but just pulling the internal “enable” jumper clip.

And here is another excellent post from KA7OEI that discusses the RX888 External Clock issue in good detail: mk2.htmlhttps://ka7oei.blogspot.com/2024/03/using-external-clock-with-rx-888-mk2.html

BeaconBlaster, FST4W, WSPR

BB-6 T/R Switch Interface

If the Beacon Blaster 6 is to be used with a receiver on a shared antenna, an external Transmit/Receive switch will be required. Some T/R switches provide automatic sensing that switches modes when the transmitter starts. This can work well, especially given the relatively low-power output of the BB-6.

But there are advantages to an externally-controlled switch, usually driven by the “PTT” output of the transmitter (this is often an “RCA” or phono jack.) This output is generally a contact-closure (relay or transistor) to ground, which connects during transmit.

The BB-6 T/R Switch Interface provides this control signal. Installation will require that a 10mm (or 3/8”) hole be drilled in the BB-6 rear panel, and the attached control wires be plugged onto an existing header on the BB-6 control board.

BB-6 firmware version 2.4.0 or later is required.

The T/R Switch Interface is offered free of charge to all owners of the BB-6, so please let me know if you want one.

More details here:

https://turnislandsystems.com/wp-content/uploads/2024/01/BB6-TRS.pdf

(the image above shows a 1/4W resistor where a small surface-mount resistor should be. Turns out I didn’t have the proper value on-hand when building the test unit.)

BeaconBlaster, FST4W, WSPR, WSPRSONDE

WSPRSONDE-8

Announcing the next stage in Beacon Blaster evolution: The WSPRSONDE 8!

This is still in development, but the initial prototypes look very promising. Similar in many ways to the BB-6, this has the same type of frequency-flexible 1W Digital (square wave) outputs, the same GPS and 10 MHz clock reference inputs, 9-24VDC input, and USB configuration port. The WSPRSONDE-8 supports both WSPR and FST4W-120 modes.

But the WSPRSONDE-8 adds these new features:

  • Eight frequency-flexible outputs, 1W, 160-6 meters (requires external filtering)
  • Spurious output levels greatly reduced. The BB-6 was designed to meet a spurious output level of -40dBc or better. The WSPRSONDE-8 uses a different modulation technique to provide close-in spurs typically better than -90dBc
  • A single-board design that eliminates the many subassemblies and interconnections inside the BB-6. This is a rugged system.

The WSPRSOND-8 provides ultimate frequency accuracy and stability limited only by the external 10 MHz reference.

WSPRSONDE-8 shown with 6-band filter/combiner and Bodnar GPSDO
Close-in spurious typically better than -90dBc (20-meters shown here, spurs -96dBc)
Using the 6-band filter/combiner, the harmonic outputs are typically better than -60 dBc (20 meter output shown here)
BeaconBlaster, FST4W, WSPR

Beacon Blaster 6 Now Supports WSPR

I’ve been running this code (or the previous version) for a few weeks now, and it runs well. The latest version of wsprdaemon is able to calculate spectral spreading for WSPR as well as FST4W, so there’s little reason not to switch over to WSPR.

There are some upgrades to the configuration commands and I will be soon editing the user’s guide to show this, but the old commands still work: just add “MODE WSPR” to the start of your config.txt file. Then reload the file (or restart the BB-6) and you’re now running WSPR!

Get the new Version 2.3.1 upload here:

https://turnislandsystems.com/downloads/

BeaconBlaster, FST4W

Eclipse!

We’ve had at least five Beacon Blaster 6 boxes running during last week’s solar eclipse (plus another custom beacon design that used the BB-6 motherboard.) A lot of good propagation data was captured using these and other beacons.

I’ve got two BB-6 prototypes running, one in Friday Harbor Washington (WB6CXC, Grid CN88) and another in Occidental California (WB6CXC, Grid CM88). These were both transmitting on 80, 40, 30, 20, 15, and 10 meters, connected through a prototype Six-Band Filter/Combiner.

Here’s the setup in Occidental:

Prior to the eclipse, in order to verify the BB-6 “baseline” performance I set up a little monitoring system. This way we can confirm the validity of remote measurements of frequency, amplitude, or spread.

To do this I used a QRP-Labs “QDX” transceiver, running as a receiver on the 20 meter band. Some external shielding and cable-clamp chokes were needed to attenuate the strong local signal, as even with a dummy-load in the QDX antenna jack the receiver was initially overloading. The QDX was then connected to a computer running the wsjtx program to decode my FST4W-120 transmissions. wsjtx saves a log file showing the time-stamped parameters of every decoded transmission.

For additional monitoring the “-40 dB Tap” output of the Combiner was connected to a spectrum analyzer, which was also plugged into the computer. I first used a “SignalHound SA-44B” analyzer, but wanting to free up that device for other measurements I switched to the amazing “tinySA” (which also has an available computer interface program.)

The Bodnar GPSDO which provides the 10 MHz clock to both the BB-6 and the QDX has a monitoring program that displays internal PLL and GPS satellite status — this was also on the screen.

Finally, the BB-6 USB serial port was connected to the “Putty” terminal emulator. Here’s how it looked:

Here’s the tinySA in action:

The BB-6 performed flawlessly!

BeaconBlaster

Beacon Blaster 6 errata

My apologies, but the help file for the “Schedule” command is in error.  Here is the corrected one:

Schedule [channel] [transmit slot] [sequence length] ([morse ID interval]) 
channel: 1-6
transmit slot : transmit on slot # 
sequence length : repeat every # timeslots 
morse ID interval : send morse ID rather than beacon every # transmissions. 
Examples: 
Sched 1 1 1 : channel 1, transmit every timeslot. 
Sched 1 1 2 : channel 1, transmit every two timeslots, starting with timeslot 1. 
Sched 6 3 3 : channel 6, transmit every three timeslots, starting with timeslot 3. 
Sched 4 1 1 5 : channel 4, transmit every timeslot, sending morse message every five transmissions. 

BeaconBlaster, FST4W

BB-6 goes to Antarctica!

The first build of the Beacon Blaster 6 transmitters is being deployed, and I am very excited to announce that one of these is on the way to the remote Neumayer Station III in Antarctica.

Some of the researchers there are radio amateurs and have been operating from this remote location using WSPR and other modes. The BB-6 will provide FST4W capability, and the frequency stability and “spreading” resolution of this mode (and this polar location) will greatly enhance the ability to study ionospheric propagation. Neumayer Station III operates under the callsign DP0GVN.

Neumayer Station III is operated by the Alfred Wegener institute, which is a center for polar and marine research, with particular focus on the cold and temperate regions of the world.

DE: Neumayer Station III in Polar Nacht. EN: Neumayer Station III in polar night.

The Alfred Wegener institute is not limited to this one Antarctic base — for more information please see:

https://www.awi.de/en/about-us.html

https://www.awi.de/en/expedition.html

https://www.awi.de/en/expedition/stations/neumayer-station-iii.html

Of particular amateur radio interest, the Alfred Wegener institute ship Polarstern was very recently operating in the vicinity of the North Pole, and has been transmitting WSPR, call sign DP0POL:

https://follow-polarstern.awi.de/?lang=en#main-menu